A filter is a device designed to pass a predetermined frequency band from an inputted RF signal. The filter has been realized in various ways. In case of an RF filter, a pass band is determined by inductance and capacitance of the filter. Tuning refers to adjusting a pass band of the filter.
In a communication system such as a mobile communication system, a plurality of pass bands are allotted to communication service providers. Generally, the service providers divide the allotted pass bands into a plurality of channels. They use a filter corresponding to allotted frequency bands.
Recently, rapid change and development of communication systems call for varying the characteristics of a filter such as center frequency and bandwidth. To meet the demand, a tunable filter has been proposed.
FIG. 1 shows a conventional tunable filter. The conventional tunable filter comprises a housing 100, an input connector 102, an output connector 104, a cover 106, and a plurality of cavities 108 and a plurality of resonators 110.
A plurality of walls are formed inside the filter and a plurality of cavities 108 are defined by the walls. Each of the resonators 110 is contained in each of the cavities. There are coupling holes on the cover 106 for coupling the cover and the housing 100. Tuning bolts 112 are inserted into the housing 100 though the cover 106. The tuning bolts 112 are inserted at or near positions where resonators are located.
An RF signal is inputted to the input connector 102 and outputted from the output connector 104. The RF signal propagates through coupling windows formed in each cavity. Resonance of the RF signal is generated by each cavity 108 and resonator 110 and filtering is performed by the resonance. In the conventional tunable filter, tuning for frequency and bandwidth is performed using the tuning bolts.
FIG. 2 is a cross sectional view of a cavity of the conventional tunable filter. Referring to FIG. 2, the tuning bolt 112 inserted though the cover 106 lies over a upper part of the resonator. The tuning bolt 112 is made of metal material and fixed to the cover 106 by a nut. The distance between the resonator 110 and the tuning bolt 112 can be adjusted by rotating the tuning bolt 112, and filter tuning is performed by adjusting the distance. The rotation of the tuning bolt 112 can be performed manually or automatically using a tuning machine.
FIG. 3 shows tuning principle in which the conventional tunable filter is tuned. Referring to FIG. 3, capacitance is generated between the tuning bolt 112 and the resonator 110. Capacitance is determined by a dielectricity, a distance, and an area between the tuning bolt 112 and the resonator 110. Capacitance is one parameter that determines center frequency of a filter. The above-described conventional tunable filter, however, has following disadvantages. When tuning is performed manually, it takes a long time because each of the tuning bolts 112 has to be rotated. This becomes severe when there are many tuning bolts because each of the tuning bolts has to be rotated independently. As a result, labor and manufacturing costs increase. Also, after tuning is performed, it is hard to lock the location of the tuning bolts. In particular, each tuning bolt must be tightly locked when a distance between the tuning bolt and the resonator is set. Tuning bolts tend to micro-rotate in the locking process, which results in failure of tuning. In order to overcome this problem, other locking means is required. In addition, it is hard to obtain a wide tuning range on account of high power trouble.
For a wide tuning range, the distance between the tuning bolt and the resonator needs to be long enough. For smaller filters, obtaining a wide tuning range is more difficult.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.